The present invention relates to magnetic particles for charging means for charging an electrophotographic photosensitive member, and an electrophotographic apparatus, a process cartridge and an image forming method using the charging means.
Hitherto, a large number of electrophotographic processes have been known. In these processes, an electrostatic latent image is formed on a photosensitive member comprising a photoconductive material by charging means and imagewise exposure means, then the latent image is developed and visualized with a toner, and the resultant toner image is, after transferred onto a transfer-receiving material, such as paper, as desired, fixed by heating, pressing, heating and pressing, etc., to obtain a copy or a print. The residual toner remaining on the photosensitive member without being transferred is removed in a cleaning step. The above steps are repeated.
In recent years, various organic photoconductive substances have been developed as a photoconductive substance for electrophotographic photosensitive member, and accordingly a function separation-type photosensitive member including a lamination of a charge generation layer and a charge transport layer is commercialized and loaded on copying apparatus, printers, facsimile apparatus, etc. In such electrophotographic apparatus, corona discharge means have been conventionally used as charging means, but are accompanied with difficulties, such as occurrence of a large amount of ozone and a filter for removing the ozone, resulting in a size enlargement and an increase in running cost of the apparatus.
As a technical solution of such difficulties, a charging method for minimizing the occurrence of ozone has been developed, wherein a charging means, such as a roller or a blade is abutted to the photosensitive member surface to form a narrow gap in the proximity of the contact portion where a discharge appearing to follow the Paschen""s law occurs. Among these, it is preferred to use a roller charging system using a charging roller as a charging means in view of the charging stability.
The charging is effected by discharge from the charging member to a charge-receiving member, so that the charging is started by application of a voltage exceeding a certain threshold. For example, in case of abutting a charging roller against a photosensitive member having a ca. 25 xcexcm-thick photosensitive layer comprising an organic photoconductor, the surface potential of the photosensitive member is started to increase by application of a voltage of ca. 640 volts or above and thereafter increased linearly proportional to an applied electric field at a slope of 1. Hereinafter, the threshold voltage is defined as a charge initiation voltage Vth. In other words, in order to obtain a surface potential Vd on the photosensitive member, a larger DC voltage of Vd+Vth has to be applied to the charging roller. Further, the resistivity of the charging roller can vary corresponding to a change in environmental conditions, so that it has been difficult to control the potential of the photosensitive member at a desired value.
For this reason, in order to accomplish a further uniform charging, it has been proposed to use a DC+AC charging system of applying to a charging roller a voltage obtained by superposing an AC voltage having a peak-to-peak voltage of at least 2xc3x97Vth on a DC voltage corresponding to a desired Vd as disclosed in Japanese Laid-Open Patent Application (JP-A) 63-149669. This aims at taking advantage of a potential smoothening effect of the AC voltage, and the potential of the charge-receiving member is converged to a central value Vd of the AC voltage, which is less affected by a change in external conditions.
In the charging method (contact or proximity charging method) based on a charging mechanism utilizing a discharge from the charging member to the photosensitive member or charge-receiving member, it is still necessary to apply a charging voltage in excess of a required surface potential of the photosensitive member. Further, as a result of application of the AC electric field, new problems have occurred, such as the occurrence of vibration of the charging member and the photosensitive member and a noise accompanying the vibration (hereinafter referred to as xe2x80x9cAC charging noisexe2x80x9d) and accelerated deterioration of the photosensitive member surface due to the discharge.
On the other hand, there has been known an image forming method wherein a photosensitive member having an electroconductive protective film is charged by using electroconductive fine particles as disclosed in JP-A 61-57958. The JP reference contains a description to the effect that a photosensitive member having a semiconductive protective film having a resistivity of 107-1013 ohm.cm can be charged uniformly, without irregularities and without causing charge-injection into the photosensitive layer by using electroconductive particles having a resistivity of at most 1010 ohm.cm, whereby good image reproduction can be accomplished. According to this method, it is possible to prevent occurrence of vibration and noise which have been problems in the AC charging, but the charging efficiency is low. Further, as the transfer residual toner on the photosensitive member is scraped by the conductive particles as the charging member, the toner is attached to the charging member, whereby the charging performance is liable to be changed.
Further, it has been desired to charge a photosensitive member by direct injection of charge.
So-called injection charging method of injecting a charge to a trap level at the surface of a photosensitive member by applying a voltage to a contact charging member, such as a charging roller, a charging fiber brush, or a charging magnetic brush has been reported in, e.g., Japan Hardcopy 92 Annual Paper Collection P. 287, xe2x80x9cContact Charging Performance by Using Electroconductive Rollerxe2x80x9d (in Japanese). According to the method, a photosensitive member which is insulating in the dark is subjected to injection charging by a low-resistivity charging member supplied with a voltage, so that the method essentially requires that the charging member has a sufficiently low resistivity and an electroconductivity-imparting substance (such as conductive filler) is sufficiently exposed to the surface. Accordingly, the above paper describes that the charging member preferably comprises an aluminum foil or an ion-conductive charging member having a sufficiently low resistivity in a high-humidity environment. According to our study, a charging member capable of effecting a sufficient charge injection to a photosensitive member may have a resistivity of at most 1xc3x97103 ohm.cm, above which a difference begins to occur between the applied voltage and the charge potential, so that the stability of charge potential is liable to be impaired.
However, when such a charging member having a low resistivity is actually used, an excessively large leakage current is liable to flow into scars or pinholes formed at the photosensitive member surface, several difficulties are caused, such as insufficient charging in the neighborhood, enlargement of the pinholes and conduction breakdown of the charging member.
In order to prevent these problems, it is necessary to provide a resistivity on the order of at least 1xc3x97104 ohm.cm to the charging member. At this level of resistivity, however, the charge injection performance into the photosensitive member is lowered, so that the effective charging cannot be performed. This is a contradiction.
Accordingly, it has been desired to solve the above-mentioned problems in a contact-type charging device or an image forming method using such a charging device. More specifically, it has been desired to satisfy in combination a good charging performance by charge injection which cannot have been accomplished without using a low-resistivity charging member and prevention of pinhole leakage on the charge-receiving member which cannot have been accomplished by using such a low-resistivity charging member.
Further, in an image forming method using a charging member contacting a charge-receiving member, the charging member is liable to be soiled (by toner melt-sticking) to cause a charging failure leading to image defects and is thus liable to cause a problem in successive image forming performance. Also in the method of directly injecting charge into a charge-receiving member, it is an urgent problem to be solved for allowing image formation on a large number of sheets to prevent the soiling of the charging member causing charging failure.
An object of the present invention is to provide magnetic particles for charging means, less liable to be soiled and capable of retaining a good charging performance for a long period.
Another object of the present invention is to provide magnetic particle for charging means, capable of showing a good injection charging performance.
According to the present invention, there is provided magnetic particles for charging means disposed in contact with an electrophotographic photosensitive member for charging the electrophotographic photosensitive member based on a voltage applied thereto, comprising a ferrite component represented by the following formula (1):
(Fe2O3)X(A)Y(B)Zxe2x80x83xe2x80x83(1),
wherein A denotes at least one metal oxide component selected from the group consisting of Li2O, MnO and MgO, B denotes at least one metal oxide component different from A; X, Y and Z denote numbers representing mol ratios and satisfying the following conditions: 0.2 less than X less than 0.95, 0.01  less than Y less than 0.5, X+Yxe2x89xa61, and 0xe2x89xa6Z less than 0.79.
According to another aspect of the present invention, there is provided an electrophotographic apparatus, comprising: an electrophotographic photosensitive member, and charging means, imagewise exposure means and developing means disposed in this order opposite to the photosensitive member, wherein said charging means includes a charging member comprising the above-mentioned magnetic particles and disposed contactable to the photosensitive member so as to charge the photosensitive member based on a voltage received thereby.
According to a further aspect of the present invention, there is provided a process cartridge, comprising: an electrophotographic photosensitive member, charging means, and, at least one member selected from developing means and cleaning means disposed, wherein
said charging means includes a charging member comprising the above-mentioned magnetic particles and disposed contactable to the photosensitive member so as to charge the photosensitive member based on a voltage received thereby, and
said electrophotographic photosensitive member, charging means and at least one member selected from developing means and cleaning means are integrally supported to form a cartridge which is detachably mountable to an electrophotographic apparatus main body.
According to a still further aspect of the present invention, there is provided an image forming method, comprising the steps of:
charging an electrophotographic photosensitive member by applying a voltage to a charging member comprising the above-mentioned magnetic particles and disposed in contact with the photosensitive member,
imagewise exposing the charged photosensitive member to form an electrostatic image on the photosensitive member, and
developing the electrostatic image.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.